By means of the micro-system technique miniaturised and cost-efficient devices may be produced which are used, for instance, in many technical fields as sensors or actuators with different functions. In particular in the automotive industry but also in mechanical engineering there is a need for complex micro-system devices fabricated in an integrated manner, which perform various measurement and control functions autonomously and with low energy requirement. Due to the continuous increase of the technological integration level of micro-systems there are attempts to fabricate micro-system devices in form of multi-sensor modules by using the so-called Wafer Level Packaging (WLP). A multi-sensor module is a sensor unit in which several single sensor modules or units are provided in a shared package or housing and which combines in itself the different functions of the single modules
When fabricating sensor or multi-sensor modules by means of wafer level packaging the structures of several sensor or multi-sensor modules are formed on a substrate wafer by corresponding coating and etch processes. The functional units of several sensor or multi-sensor modules rest on the substrate wafer in a matrix-like manner side-by-side and/or on top of each other with intermediately provided isolation sections. The substrate wafers are connected to correspondingly produced cap wafers so that each of the (multi) sensor chips is firmly connected with a corresponding package chip. The composite consisting of the substrate wafer and the cap wafer is separated into individual chips, i.e., into the individual sensor or multi-sensor modules, after the connection on wafer level
Due to the extensively parallel production technique the packaging on wafer level has enormous advantages compared to packaging on chip level with respect to costs, device integration density and yield.
One essential issue in the production of multi-sensor modules using WLP is the fact that, for fulfilling their respective functions, different sensor systems require individual operating pressures and/or individual gas compositions which in part significantly differ from each other. For example, resonant systems mostly have a high quality, that is why they are operated at low operating pressures. Resonant rotation rate sensors are typically operated at an operating pressure of several μbar up to several mbar, in order to avoid an undesired strong damping by the gas surrounding the sensor or its detection unit. Acceleration sensors that are based on the principle of inertia have usually to be damped strongly so that in this case typical operating pressures are of several 100 mbar. The following table illustratively shows respective typical operating pressures for various micro-systems:
Sensor/device typeOperating pressureAcceleration sensor300-700mbarAbsolute pressure sensor1-10mbarResonant sensor0.1mbar(e.g. rotation rate sensor)Bolometer<0.0001mbarOscillator<0.0001mbar
Due to the parallel processing mode in WLP the establishment of corresponding pressures or gas compositions in the various cavities of single wafer to be separated into the individual modules represents an essential difficulty.
Applicant of the present invention has developed a method to integrate several sensor units requiring different operating pressures and/or gas compositions in a multi-sensor module, in which method cavities having defined and possibly different gas pressures and/or gas compositions may be formed during the WLP in a single process step. In this case, the assembly of substrate wafer and cap wafer is accomplished in a process chamber that may be provided with a gas or a gas mixture of a corresponding composition and at a corresponding pressure. Due to production process requirements first in each cavity of the single wafer the same gas composition at equal pressure is adjusted during the WLP. The gas pressure and/or the gas composition of different cavities are then differently established by using getter materials in selected cavities. However, this process technique does not enable an arbitrary adjustment of different pressure values and/or gas compositions in the respective cavities according to the present state of the art. If, for example, a resonant rotation rate sensor is to be operated in a first cavity at a pressure of about 0.1 mbar, then the maximum pressure that may be generated in other cavities of the single wafer using the above-described process technique is about 200 mbar. This pressure, however, is too low for a precise and reliable operation of acceleration sensors, a damping of vibrations can be ensured in an insufficient manner only.